THE PRINCIPLE OF VITAL AFFINITY. 175 
of almost all vegetables,—that earth existing in large quantity in all fertile soils, 
whereas it is “ very rarely found in the ashes of plants.” 
In the animal kindom the same power of simple selection and extraction is 
more fully exemplified, perhaps most strikingly in the development of many of 
the lower classes, of which the organization is simple, and the matters deposited 
from the nourishing fluid remarkably diversified, as in many of the radiata and 
mollusca, which have horny and earthy integuments. And in all animals, so far 
as any chemical change is effected in the vital actions of absorption, secretion, 
and even nutrition, it would appear to be chiefly of this simple kind, consist- 
ing in the selection and appropriation of compounds already existing in the 
fluids on which these functions are performed, not in the formation of new 
compounds. The chyme which is found in the intestines of an animal during 
digestion contains all the compounds (albuminous, fatty, and extractive matters) 
which are found in the chyle absorbed from it, although these are in a different 
state of aggregation, and associated also with other matters which are not ab- 
sorbed. Since it has been ascertained that the compounds which used to be 
thought peculiar to the greatest secretions in the body, the bile and the urine, 
pre-exist in the blood, and are only evolved at the liver and kidneys,—accumu- 
lating, therefore, in the blood, when the secretive action of these organs is sus- 
pended,—it has become obvious that the main office of these organs is not forma- 
tive, but only attractive, to extract from the blood compounds already existing 
there. And, although there is one material extensively employed in the forma- 
tion of animal textures, viz., gelatin, which cannot be detected in the blood; yet, 
as this is the only material so employed which cannot be found there, and as a 
substance very closely resembling it is found there under certain circumstances, 
Wwe may assert that in animals by far the greater part of the act of nutrition, 
numerous and diversified as the compounds forming the solid materials of animal 
bodies may be, is likewise of this simple kind. 
We may consider, then, the selection and extraction, from a previously exist- 
ing compound fluid, by the agency of a previously existing compound solid, of 
certain portions of that fluid already elaborated, as a chemical action, essential 
to all living beings, and so peculiar to them that it may be, at least with high 
probability, termed an exercise of a vital affinity. And, in regard to this simplest 
kind of such action, the following points may be considered as ascertained :— 
1. It seems to be always performed, in the perfect vegetable or animal, by 
an agency, not of vessels, as was formerly supposed, capable of a vital contraction, 
_and of changing the nature of their contents by the degrees of that contraction, 
but of cells, either pre-existing in the solid structure, or carried about in the 
nourishing fluid, and having the name of the globules or corpuscules of that 
fluid. Most of the textures seem to be formed by the gradual transforma- 
tion, elongation, or flattening of cells, which have sprung from nuclei at- 
